This invention relates to optical recording media.
For optical recording media of magneto-optical memory type, there are well known a number of materials for a recording layer thereof, for example, MnBi, MnAlGe, MnSb, MnCuBi, GdFe, TbFe, GdCo, PtCo, TbCo, TbFeCo, GdFeCo, TbFeO.sub.3, GdIG (gadolinium iron garnet), GdTbFe, GdTbFeCoBi, CoFe.sub.2 O.sub.4, etc. These materials are deposited on transparent substrates of plastic material or glass as a thin film by any suitable thin-film forming techniques such as vacuum deposition or sputtering. The features common to these magneto-optical recording thin film layers are that the axis of easy magnetization is perpendicular to the film surface and that Kerr and Farady effects are great.
Requirements imposed on such magneto-optical recording media are:
(1) that the Curie point is of the order of 100.degree. to 200.degree. C. and the compensation point is close to room temperature, PA1 (2) that noise-inducing defects such as grain boundaries are relatively fewer, and PA1 (3) that a magnetically and mechanically uniform film is obtained over a relatively large area.
In the light of these requirements, a great attention is recently drawn to amorphous perpendicular magnetizable thin films of rare earth element-transition metal among the above-mentioned materials. Magneto-optical recording media having such amorphous perpendicular magnetizable thin films of rare earth element-transition metal, however, have a storage problem. If the magnetic thin film layers are stored in contact with the ambient atmosphere, rare earth elements therein are preferentially erroded or oxidized by oxygen and moisture in the atmosphere, losing the necessary information recording and reproducing ability. The rotational angle available upon reading of recorded signals should be as large as possible in order to improve the S/N ratio.
For this reason, most investigations are generally directed to those recording media of the construction having an intermediate layer disposed on a surface of a magnetic thin film layer adjacent to or remote from the substrate. The intermediate layer is provided for the purpose of imparting corrosion resistance or moisture proofness and adding a multiple interference effect or Farady effect to Kerr effect to increase the rotational angle. Known intermediate layers are vacuum deposited films of inorganic materials such as silicon monoxide, silicon dioxide, aluminum nitride, silicon nitride and zinc sulfide as well as resinous coatings (see Japanese Patent Application Kokai Nos. 58-80142 and 59-52443). However, these layers are insufficient in corrosion resistance or the like.
It is also known to form an intermediate layer from a mixture of oxide and nitride. For example, Japanese Patent Application Kokai No. 60-145525 discloses a mixture of Si.sub.3 N.sub.4 and SiO.sub.2, Si.sub.3 N.sub.4 and SiO, or AlN and Al.sub.2 O.sub.3.
These intermediate layers, however, are not satisfactory with respect to corrosion resistance, C/N (carrier-to-noise ratio), delamination, cracking, initial deformation of the medium like warpage, and film forming speed. There is a need for an optical recording medium having more improved properties.
The same problem arises in an optical recording medium having a recording layer of the so-called phase conversion type.